Interpretation of mutants in leaf morphology: genetic evidence for a compensatory system in leaf morphogenesis that provides a new link between cell and organismal theories.

On the basis of "cell theory," we tend to think that some changes in cellular behavior must be responsible for mutant morphology. According to the cell theory, the unit of morphogenesis of a multicellular organism is the cell. Another interpretation of morphogenesis of plants is the "organismal theory," which postulates that the individual cell is not the basic unit of morphogenesis. Here we examine the validity of the cell and organismal theories, with particular emphasis on the phenotypes of mutant or transgenic Arabidopsis plants with altered leaf morphology. Genetic evidence shows that a compensatory system(s) is involved in leaf morphogenesis, and an increase in cell volume might be triggered by a decrease in cell number. Such evidence provides a new link between cell and organismal theories. In conclusion, the size and number of leaf cells affect the dimensions and sizes of leaves. Moreover, the leaf size is, at least to some extent, uncoupled from the size and number of cells by the compensatory system(s).